1. Technical Field
This invention relates generally to measurement circuits, and more specifically to a circuit for auto-calibration of measurement circuits including analog to digital converters.
2. Background Art
Many electronic devices need to make accurate measurements to operate properly. For example, a household thermostat needs to be able to measure the temperature to properly operate a furnace or air conditioner. When a person sets the heat at 68xc2x0, they expect the heat to come on when the temperature of the room falls below 68xc2x0 If the thermostat""s xe2x80x9cthermal measurement systemxe2x80x9d, which is technical language for a xe2x80x9cthermometerxe2x80x9d, cannot measure the room temperature accurately, it never knows exactly how cold the room is. As can be seen, proper measurement is critical to the thermostat""s operation.
Likewise, in electronic circuits, there are many needs for measurement accuracy. One such example is a cellular phone battery. Many cellular phones today have xe2x80x9cgas gaugesxe2x80x9d on the screens that tell you how much battery power you have left. Additionally, lithium and nickel-based battery chargers generally measure the battery voltage to decide when to stop charging. Thus, battery measurement is a critical system in battery charging.
Many systems today use integrated circuits to measure battery voltage. These circuits often include an analog to digital (A/D) converter. An A/D converter reads an analog level, like a battery voltage for example, and converts it into a digital word that a microprocessor, microcomputer or other digital circuitry can understand.
One problem with A/D converters is that they must be calibrated. While they are very good at measuring differential changes, they need to be calibrated to measure absolute magnitudes. In other words, while a digital thermometer is very good at measuring a change from 75 to 80 degrees, a calibration is required to tell the thermometer exactly where 80 degrees occurs.
By way of example, when you purchase wristwatch, you must first manually set the watch to the correct time. This is calibration. From that time on, the watch measures the time accurately. A/D converters must be initially calibrated in the same fashion.
Another problem with A/D converters is that from time to time the calibration may drift. This is analogous to a watch running a little fast or a little slow. Consequently, the A/D converter must be recalibrated occasionally to keep the readings accurate.
One prior art solution for calibration is to occasionally measure the ground voltage and set the digital output to zero. This approach has two inherent problems. First, the ground potential often includes quite a bit of noise. Consequently, what the A/D converter thinks is zero may actually be half a volt or minus half a volt.
Second, the ground measurement often does not take into consideration the electrical components. For example, if a filter circuit is between ground and the A/D input, tolerances in the components will not be measured when ground is measured.
There is thus a need for an improved auto-calibration circuit and method.